docs: def _format_variables(self, raw_vars):
          """
          :param raw_vars: a `dict` of `var_name: var_object` pairs
          :type raw_vars: dict
          :return: sorted list of variables as a unicode string
          :rtype: unicode
          """
          f_vars = []
          for var, value in raw_vars.items():
              if not (var.startswith('__') and var.endswith('__')):
                  repr_value = self._get_repr(value)
                  f_vars.append('{0} = {1}'.format(var, repr_value))
          return '\n'.join(sorted(f_vars))